Improve the ergonomics of creating local buffers (#10347)

This PR renames `language::Buffer::new` to `language::Buffer::local` and
simplifies its interface. Instead of taking a replica id (which should
always be 0 for the local case) and a `BufferId`, which was awkward and
verbose to construct, it simply takes text and a `cx`.

It uses the `cx` to derive a `BufferId` from the `EntityId` associated
with the `cx`, which should always be positive based on the following
analysis...

We convert the entity id to a u64 using this method on `EntityId`, which
is defined by macros in the `slotmap` crate:

```rust
    pub fn as_ffi(self) -> u64 {
        (u64::from(self.version.get()) << 32) | u64::from(self.idx)
    }
```

If you look at the type of `version` in `KeyData`, it is non-zero:

```rust
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct KeyData {
    idx: u32,
    version: NonZeroU32,
}
```

This commit also adds `Context::reserve_model` and
`Context::insert_model` to determine a model's entity ID before it is
created, which we need in order to assign a `BufferId` in the background
when loading a buffer asynchronously.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

crates/vim/src/editor_events.rs

@@ -73,14 +73,14 @@ mod test {
     use crate::{test::VimTestContext, Vim};
     use editor::Editor;
     use gpui::{Context, Entity, VisualTestContext};
-    use language::{Buffer, BufferId};
+    use language::Buffer;
 
     // regression test for blur called with a different active editor
     #[gpui::test]
     async fn test_blur_focus(cx: &mut gpui::TestAppContext) {
         let mut cx = VimTestContext::new(cx, true).await;
 
-        let buffer = cx.new_model(|_| Buffer::new(0, BufferId::new(1).unwrap(), "a = 1\nb = 2\n"));
+        let buffer = cx.new_model(|cx| Buffer::local("a = 1\nb = 2\n", cx));
         let window2 = cx.add_window(|cx| Editor::for_buffer(buffer, None, cx));
         let editor2 = cx
             .update(|cx| {
@@ -115,7 +115,7 @@ mod test {
         let mut cx1 = VisualTestContext::from_window(cx.window, &cx);
         let editor1 = cx.editor.clone();
 
-        let buffer = cx.new_model(|_| Buffer::new(0, BufferId::new(1).unwrap(), "a = 1\nb = 2\n"));
+        let buffer = cx.new_model(|cx| Buffer::local("a = 1\nb = 2\n", cx));
         let (editor2, cx2) = cx.add_window_view(|cx| Editor::for_buffer(buffer, None, cx));
 
         editor2.update(cx2, |_, cx| {